Removal of CO2 from gas mixtures

ABSTRACT

An improved, catalyzed process for the removal of CO 2  from gas mixtures is described using a solution containing 15-40% by weight of potassium carbonate in which the absorption efficiency is enhanced by the addition of sodium or potassium vanadate equivalent to 2 to 10% by weight of V 2  O 5  and sodium or potassium borate equivalent to 1 to 10% by weight of KBO 2  in which the weight ratio of equivalent KBO 2  to equivalent V 2  O 5  is less than 1.5:1. It has been found that use of the described catalysts permits significant reductions in solution circulation rates (up to 45%), which leads to equipment and utility economies.

This invention relates to a process for the removal of CO₂ from gasmixtures by means of aqueous solutions of potassium carbonate containingpotassium vanadate and borate additives in a defined ratio and withconcomitant reductions in scrubbing solution flow rates.

The removal of CO₂ from gas mixtures with aqueous potassium carbonatesolutions is well known. The absorption is generally done with a hotsolution as described in U.S. Pat. No. 2,886,405. The efficiency of theprocess is greatly enhanced by the use of amine and especially by amineborate additives described in U.S. Pat. Nos. 4,271,132 and 3,851,041 ofA. G. Eickmeyer.

However, such organic additives are subject to oxidative degradationwhen scrubbing gases containing oxygen, especially at elevatedtemperatures. This causes the solution to lose absorption efficiency andto accumulate undesirable oxidation products. For the same reasons suchsolutions cannot be regenerated by means of air stripping.

In addition, the equipment and utility costs associated with CO₂ removalplants are considerable; therefore, workers in the art are continuouslysearching for ways to lower such costs. One possibility is to lower thecirculation rate of the scrubbing solution, which saves money onequipment (e.g., smaller pumps and towers) and also on steam in theregeneration section. Obviously, however, such cost saving measures canbe effected only if the scrubbing solution, at design concentrations andtemperatures, is capable of handling the gas removal load at the loweredcirculation rates.

Gas mixtures for which the present invention is well suited includerecycle gas in the production of ethylene oxide from ethylene andoxygen, flue gas and lime kiln gas.

U.S. Pat. No. 3,907,969 of Joseph H. Field discloses aqueous solutionsof potassium carbonate containing KBO₂ and V₂ O₅ in which the weightratio of equivalent KBO₂ to equivalent V₂ O₅ is at least 1.5:1. Thetests described in this patent indicate that the CO₂ absorption rate isimproved by the addition of both KBO₂ and V₂ O₅ but that the CO₂ pickupof the solution is increased by the KBO₂ and decreased by the V₂ O₅.Therefore the weight ratio of KBO₂ to V₂ O₅ is specified to be at least1.5:1. Indeed, in Table IV of this patent results are set forth whichdemonstrate that, at KBO₂ /V₂ O₅ ratios of less that 1.5:1 CO₂ pickup isdecreased as compared with the control.

Surprisingly, it has been discovered that both the absorption rate andthe CO₂ pickup are increased by increasing additions of V₂ O₅ topotassium carbonate solution and that it is actually preferable to havea weight ratio of KBO₂ to V₂ O₅ of less than 1.5:1. Further, the systemsof the invention can be advantageously operated at circulation ratessignificantly below the rates employed with a comparable (i.e., the sameK₂ CO₂ concentration and temperature (non-catalyzed hot potassiumcarbonate solution, when scrubbing identical gas streams. Such reductioncan be up to 45%, and preferably from about 30 to 40%.

The scrubbing solutions hereof use K₂ CO₃ as the principal salt inconcentrations from 15% to 40% by weight and preferably from 20% to 30%by weight. The solutions are usually cycled between an absorption stage,where CO₂ is picked up by the solution from the gas, and a regenerationstage, where CO₂ is desorbed from the solution by steam or airstripping. The absorption is preferably conducted at superatmosphericpressure and the regeneration is preferably at near atmosphericpressure. The absorption temperature may be substantially the same asthe regeneration temperature. In some cases a small amount of cooledsolution may be sent to the top of the absorber in order to remove CO₂down to a low level.

Some CO₂ remains dissolved even in the regenerated solution, where it ispresent at KHCO₃. Thus a regenerated solution of 25% equivalent K₂ CO₃with 30% of the K₂ CO₃ converted to KHCO₃ would have 17.5% K₂ CO₃ and10.85% KHCO₃ by weight.

Sodium or potassium vanadate is added to the solutions in an amount from2% to 10% by weight of equivalent V₂ O₅. The preferred concentration isfrom 3% to 8% by weight equivalent V₂ O₅. The vanadate may be added asthe sodium, potassium or ammonium meta vanadate or it may be formed inplace by dissolving V₂ O₅ in the K₂ CO₃ solution as follows:

    K.sub.2 CO.sub.3 +V.sub.2 O.sub.5 →2 KVO.sub.3 +CO.sub.2

Also it may be added as sodium or potassium pyrovanadate, Na₄ V₂ O₇ orK₄ V₂ O₇, or it may be preformed by reacting V₂ O₅ with KOH solution.

Taking into account the equivalent weights, the addition of 1.52% byweight of KVO₃ or 1.34% NaVO₃ gives 1% by weight of equivalent V₂ O₅.

The effective amount of sodium or potassium borate in the scrubbingsolutions of the invention is in the range of from 1% to 10% by weightof equivalent KBO₂ and the preferred range is from 2% to 8% by weight ofequivalent KBO₂. The borate may be added as sodium or potassiummetaborate, Na₂ B₂ O₄ or K₂ B₂ O₄, or sodium or potassium tetraborate,Na₂ B₄ O₇, or K₂ B₄ O₇.8H₂ O, or as other borate salts. Also it can beformed by adding boric acid to potassium carbonate solution as follows:

    2 H.sub.3 BO.sub.3 +K.sub.2 CO.sub.3 →2 KBO.sub.2 +CO.sub.2 +3H.sub.2 O

In this case the addition of 0.755% by weight of H₃ BO₃ gives 1% byweight of equivalent KBO₂.

In typical systems of the invention, the temperature of the absorptionsolution is maintained within the range of 180°-250° F.; typicalpressures within the absorption stage range from 200-500 psig.

EXAMPLES

The relative rates of CO₂ absorption and the relative pickup of CO₂ wasmeasured at 80° C. using 25% by weight of aqueous K₂ CO₃ solutionwithout and with various amounts of V₂ O₅ and boric acid added to givethe specified percentages of V₂ O₅ and equivalent KBO₂ shown on Table I.Percentage reductions in circulation rate when using the compositions ofthe invention as compared with a non-catalyzed control were alsodetermined.

                                      TABLE I                                     __________________________________________________________________________                               CALCULATED                                                                    PERCENTAGE REDUCTION                               Run                                                                              K.sub.2 CO.sub.3                                                                  V.sub.2 O.sub.5                                                                   KBO.sub.2                                                                         RATE                                                                              CO.sub.2 PICKUP                                                                       IN CIRCULATION RATE                                __________________________________________________________________________    A  25  0   0   1.00                                                                              1.00    --                                                 B  25  2   4   2.91                                                                              1.45    31%                                                C  25  4   2   3.79                                                                              1.54    35%                                                D  25  8   2   6.21                                                                              1.71    42.5%                                              E  25  8   8   5.21                                                                              1.63    38.7%                                              __________________________________________________________________________

The relative rate of CO₂ absorption was determined by comparing thepseudo first order absorption rate, that is the natural logarithm of theratio of the CO₂ entering to the CO₂ leaving, for solutions having thesame equilibrium back pressure. The increased CO₂ pickup of the moreactive solutions is believed to result from better regeneration of thelean solution and a closer approach to equilibrium for the richsolution, thus giving a higher solution conversion range or "pickup."

Comparing runs B and C shows that a 2:1 ratio of V₂ O₅ to KBO₂ (run C)rather than a 2:1 ratio of KBO₂ to V₂ O₅ (run B) gives an appreciableincrease in the relative absorption rate as well as the CO₂ pickup.Comparing runs C and D shows that doubling the V₂ O₅ content, whileholding the KBO₂ the same, enhances the solution effectivenessappreciably. Comparison of runs D and E shows that increasing the KBO₂with the same V₂ O₅ decreases the solution effectiveness somewhat. Also,as set forth in the righthand column, the solutions permit verysignificant reductions in circulation rates and therefore give theattendant advantages noted above.

Since the solutions hereof are not subject to oxidative degradation theyare well suited to removal of CO₂ from gases also containing oxygen,such as ethylene oxide recycle gas, flue gas and lime kiln gas. For thesame reason it is practical to regenerate the solutions by means of airstripping or a combination of air and steam stripping.

I claim:
 1. In a method for absorbing CO₂ from a gas mixture containingthe same wherein a hot absorption solution having from 15 to 40% byweight of potassium carbonate is continuously circulated at acirculation rate through an absorption stage and a regeneration stage,said gas mixture is contacted with said solution in said absorptionstage to absorb CO₂ from the mixture, and said solution is thereafterregenerated in said regeneration stage by removal of CO₂ therefrom, theimprovement which comprises the steps of:adding to said solution fromabout 2 to 10% by weight equivalent V₂ O₅ and from about 1 to 10% byweight of equivalent KBO₂, the weight ratio of equivalent KBO₂ toequivalent V₂ O₅ being less than 1.5:1; and reducing said circulationrate by an amount of up to about 45%.
 2. The method of claim 1, saidcirculation rate being reduced by an amount of about 30 to 40%.
 3. Themethod of claim 1 wherein the concentration of equivalent V₂ O₅ is fromabout 3 to 8% by weight, and the concentration of equivalent KBO₂ isfrom about 2 to 8% by weight.
 4. The method of claim 1 wherein theweight ratio of equivalent KBO₂ to equivalent V₂ O₅ is less than 1:1. 5.The method of claim 1 wherein the concentration of potassium carbonatein said solution is from about 20 to 30% by weight.
 6. The method ofclaim 1, said absorption being carried out at superatmosphericpressures.
 7. The method of claim 1, said regeneration including thesteps of steam stripping said solution.
 8. The method of claim 1, thetemperature of said solution during said absorption step beingapproximately the same as the temperature thereof during saidregeneration step.
 9. The method of claim 1, said solution being at atemperature of from about 180° to 250° F.